ETH Zurich: researchers create an optical transistor from a single molecule

Researchers at ETH Zurich have moved one step closer to an optical computer, creating an optical transistor with a single molecule.

Scientists have been trying for some time to find ways to produce integrated circuits that operate on the basis of photons instead of electrons. This is not only because photons generate much less heat than electrons, but they also enable considerably higher data transfer rates.

Although a compact optical transistor is still a long way off, Vahid Sandoghdar, Professor at the Laboratory of Physical Chemistry of ETH Zurich, has achieved a decisive breakthrough by successfully creating an optical transistor with a single molecule. The researchers made use of the fact that a molecule’s energy is quantized. When laser light strikes a molecule that is in its ground state, the light is absorbed. As a result, the laser beam is quenched. Conversely, it is possible to release the absorbed energy again in a targeted way with a second light beam. This occurs because the beam changes the molecule’s quantum state, amplifying the light beam.

This so-called stimulated emission, which Albert Einstein described over 90 years ago, also forms the basis for the principle of the laser. However, amplification in a conventional laser is achieved by an enormous number of molecules. By focusing a laser beam on only a single tiny molecule, the ETH Zurich scientists have now been able to generate stimulated emission using just one molecule. They were helped in this by the fact that, at low temperatures, molecules seem to increase their apparent surface area for interaction with light. The researchers therefore needed to cool the molecule down to minus 272 degrees Celsius, or one degree above absolute zero. In this case, the enlarged surface area corresponded approximately to the diameter of the focused laser beam.

Using one laser beam to prepare the quantum state of a single molecule in a controlled fashion, scientists could significantly attenuate or amplify a second laser beam. This mode of operation is identical to that of a conventional transistor, in which electrical potential can be used to modulate a second signal. Thus component parts such as the new single molecule transistor may also pave the way for a quantum computer.